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BIG PHYSICS, BIG QUESTIONS –

Quarter of California’s snowpack loss is from human-made warming

High-level problem

David McNew/Getty Images

By Chelsea Whyte

In 2015, after four years of drought, the snowpack in the Sierra Nevada mountains of California hit a record low. Global warming is to blame for a quarter of that loss, a study based on climate models suggests.

As the planet continues to warm, more than half the Sierra snowpack is likely to vanish over the next century. “We found pretty grim results,” says Neil Berg of research institution the Rand Corporation. Simply having higher temperatures is devastating for the build-up of wintertime snowpack in the Sierras, he says.

This is bad news for California as the snowpack provides a third of its water. The state’s water infrastructure relies on snow building up in the mountains in winter and then gradually melting throughout the summer.

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In the future, overall precipitation levels in the mountains will stay the same, the study suggests. But without ways to store the extra water flowing down from the mountains during the warmer winters – and with higher losses from evaporation in summer too – the state’s problem with water shortages could get much worse.

Berg and his team compared the period of extreme drought between 2011 and 2015 with a simulated situation where there was no warming due to human emissions of greenhouse gases. The results suggest that climate change exacerbated snowpack loss in the Sierra Nevadas.

Sensitive snow

At elevations of 1500 to 2500 metres, which sit at the freezing line and are therefore more sensitive to small changes in temperature, up to 43 per cent of the snowpack loss could be due to global warming.

This year, after months of precipitation caused by “atmospheric rivers” in California, the snowpack in the Sierras is the biggest it has been in 22 years. But that doesn’t mean it will last.

The models suggest snowpack loss in the Sierras will be severe even in the most optimistic scenario for future warming. “There’s no elevation that’s totally untouched here. There are significant reductions even at the highest elevations in the Sierras,” says Berg, who led the study while at the University of California, Los Angeles.

One solution could be to capture the extra winter run-off by injecting it into groundwater reserves, says Noah Diffenbaugh, a climate scientist at Stanford University, California. This groundwater recharging would be more cost-effective than building more dams or desalinating seawater, he says.

“The other benefit is that it would allow the state to relieve pressure on the reservoirs. Instead of having constant tension between storage for later in the season versus letting out water for flood control, the groundwater storage would make more room available for flood control,” he says.